Stability of Completely Filled and Half Filled Subshells

Stability of Completely Filled and Half Filled Subshells

The electronic configuration of an atom determines many of its properties, but the stability of a particular configuration is what governs which configuration is actually adopted. You have seen that electrons fill orbitals according to the Aufbau principle, Hund's rule, and the Pauli exclusion principle. But why do certain configurations — especially those with exactly half-filled or completely filled subshells — appear so frequently and confer exceptional stability?

The answer lies in symmetry and exchange energy. When electrons occupy orbitals of the same energy (degenerate orbitals), they arrange themselves to maximise the total spin. A half-filled subshell (each orbital containing one electron with parallel spins) or a completely filled subshell (each orbital containing two electrons with opposite spins) represents a state of maximum symmetry. This symmetry leads to extra stability through what is called exchange energy. …